Patent GB-683899-A (GB)
Title: Improvements relating to graphical recording systems
Inventor: WILLIAMSON DAVID THEODORE NELS
IPC: H04N1/024
Filed: 1949-11-08
Abstract: 683,899. Electric control systems. FERRANTI, Ltd. Feb. 2, 1951 [Nov. 8, 1949], No. 28530/49. Class 40 (i) [Also in Groups XIX, XL(b) and XL(c)] A graphical recording system, Fig. 2, comprises a sheet 10 of electro-responsive material and a line of stylus members 22 which co-operate with an earthed roller 29 located beneath the material to cause visible indications in the material when electrically energized. The stylus members are each associated with a selective means responsive to an electrical signal having a frequency or amplitude individual to that member. As shown in Fig. 1 each stylus is connected to a tuned circuit L<SP>1</SP> C<SP>1</SP> ... Ln Cn, the circuits being tuned to separate frequencies forming an arithmetic series within a range of, say, 1 to 2 Mc/s, and connected in parallel so as to be energized by signals passed through amplifier 36. To record the waveform of a voltage varying with time, the voltage source 38 is applied to a reactance modulator 37 to cause the frequency of an oscillator 39 to vary in accordance with the voltage. The recording sheet is drawn at a steady rate under the stylus members and the variable frequency signal is applied through the amplifier to the tuned circuits. As the frequency varies, selected ones of the stylus members become energized to cause a series of discrete marks 41 which reproduce the waveform. The waveform of a fluid pressure varying with time is recorded by applying the pressure to displace a diaphragm with respect to an electrode, Fig. 3 (not shown), whereby a variation of capacitance is obtained to modulate the oscillator frequency. A mechanical waveform is recorded by employing a linkage to displace the core of an inductance controlling the oscillator frequency, Fig. 4 (not shown). If the amplitude of the waveform is insufficient to vary the frequency over the full range of tuned circuits L' C'-Ln Cn, the waveform is applied to vary the frequency of a very high frequency oscillator which is then hetrodyned with a fixed frequency oscillator to produce different frequencies within the range of the tuned circuits, Fig. 5 (not shown). More than one waveform may be recorded simultaneously, one oscillator being employed for each waveform and the outputs being applied in common to amplifier 36. The stylus members need not be equally spaced and the frequency values of the tuned circuits need not form an arithmetic series. Such variations may be employed to correct for constant distortion. In Fig. 14, each of the tuned circuits is replaced by a biased transformer ST', ST<SP>2</SP> ... responsive to a voltage having an amplitude individual to the device. Each transformer comprises a core carrying primary windings P', P<SP>2</SP> ... connected to an alternating source 110 and secondary windings S', S<SP>2</SP> ... connected to the respective stylus members. The output from amplifier 36 is applied in series to signal bias windings SB', SB<SP>2</SP> ... and each core carries a local bias winding LB<SP>1</SP>, LB<SP>2</SP> ... wound in opposition to the signal bias windings and connected to a D.C. source 111. As shown in Fig. 15, which illustrates the conditions existing in three transformers ST<SP>1</SP>... ST<SP>3</SP>, the local bias windings are arranged to produce a level of saturation H' ... H<SP>3</SP> different for each transformer. Where the windings are connected in series to a common source as shown, this is achieved by employing a different number of turns for each transformer. The dotted lines 121, 122 indicate the limits of the alternating excitation produced by the primary windings. With zero level of signal from amplifier 36, none of the transformers illustrated produces an alternating output in its secondary since all are biased beyond saturation. When however a signal corresponding to a magnetising level of, say, H<SP>2</SP> is passed through the local bias windings, transformer ST<SP>2</SP> is brought to the effective part of the B/H curve and a secondary output is obtained to energize the associated stylus. The signal is of insufficient amplitude to render ST<SP>1</SP>, effective and overbiases ST'. If therefore a voltage waveform to be recorded is applied to amplifier 36, different ones of the transformers ST<SP>1</SP>, ST<SP>2</SP> become effective at different amplitude levels as the voltage varies, the associated stylus members marking the recording material to reproduce the waveform. An adjustable direct current may be superposed on the output of amplifier 36 to reproduce an axis for the waveform. Where amplifier 36 is a D.C. amplifier the D.C. component normally present in the output may reproduce the axis. The zero axis may also be recorded by omitting the local bias winding of the transformer for the relevant stylus and then winding in opposite direction either the local bias or signal bias windings of the transformers for the stylus members respectively above and below the zero axis. Fig. 16 (not shown) illustrates a modification in which the secondaries energize the stylus members through biased valves arranged to produce pulsating direct current outputs suitable for use where the recording medium responds to electrolytic action. Modifications involve the use of a recording medium responsive to H.F. heating, and the use of some intermediate agency such as relays between the selective devices and the stylus members.